Polyamides



acid mixtures or above mentioned Patented May 4,1948

. 2,44i,os1

POLYAMIDES Theodore Le Sueur Cairn gaivgnor to E. I. du Pont s, Wilmington, Del., asde Nemours & Company,

11, Del., a corporation oi. Delaware No Drawing. Application October 18, 1944,

Serial No. 559,252

7 Claims.

This invention relates to the field of synthetic linear polyamides and more particularly to the process for. obtaining an improved N-alkoxymethyl polyamide.

This case is a. continuation-impart of my application Serial No. 507,745, filed October 26, 1943. now abandoned.

Highmolecular weight synthetic linear polyamides are a well known class of polymers of considerable commercial importance. They are usually prepared by the condensation of substantially equimolecular amounts of a diamine and a dibasic acid or an amino acid or mixtures of these compounds as has been described in United States Patents 2,071,250, 2,071,253 and 2,130,948. The polyamides of this kind, generally speaking, comprise the reaction product of a linear polymer-iorming composition containing amideforming groups, for example, reacting material consisting essentially of bifunctlonal molecules each containing two reactive groups which are complementary to reactive groups in other molecules and which include complementary amideforming groups. These polyamides can be obtained by the methods given in the above mentioned patents and by other methods, for example, by self-polymerization of a monoaminomonocarboxylic acid, by reacting a diamine with a dibasic carboxylic acid in substantially equimolecular amounts, or by reacting a monoaminomonohydric alcohol with a dibasic carboxylic acid in substantially equimolecular amounts, it being understood that reference herein to the amino acids, diamines, dibasic carboxylic acids, and amino alcohols is intended to include the equivalent amide-forming derivatives of these reactants. These linear polyamides include also polymers, as for instance the polyester-amides, obtained by admixture of other linear polymeri orming reactants, as for instance glycol-dibasic hydroxy acids, with the polyamide-forming reactants. The polymers of the kind which are used in the practice of this invention, having an average number of carbon atoms of at least 2 separating the amide groups and are fiber-forming or high molecular weight products as evidenced by their intrinsic viscosity which is at least 0.4 as defined in the above-mentioned Patent 2,130,948.

Substantially all of the above described linear polyamides, as exemplified by the most valuable members, such as polyhexamethylene adipamide or polyhexamethylene sebacamide, or interpolyamides are insoluble in common organic solvents. a new class of nitrogen-substituted polyamide prepared from the above mentioned polyamides by treatment with formaldehyde and an alcohol in the presence of an oxygen containing acid have been found to produce polymers having many desirable properties, including solubility in such organic solvents as alcohol. This process has been described in my copending application Serial No. 445,635, filed June 3, 1942, now abandoned, in which the products obtained are shown to be N-alkoxymethyi polyamides, namely, polyamides of the above mentioned kind in which the functional groups contained as an integral part of the polymer chain have been converted to groups or the formula in which R is a hydrocarbon radical. The N- alkoxymethyl polyamides obtained by this process, that is, by treatment of a synthetic linear polyamide which has hydrogen bearing carbonamide groups with formaldehyde and an alcohol in the presence of an oxygen containing acid, usually have in addition to the N-alkoxymethyl group, some carbonamide groups which have undergone no change and also some which have N-methylol groups. The oxygen containing acid should have an ionization constant at least as great as 9.6 x 10- and an equivalent conductance, measured at 25 C., in 0.01 N concentration, no greater than 370 ohmscmfi. These N-alkoxymethyl polyamides have been found to have good solubility in many organic solvents. However, their application for many uses requiring good heat or melt stability, such as for calendering or injection molding, is not entirely satisfactory. Prolonged heat treatment of the polymers obtained in the manner described above such as in calendaring or similar operations results in the formation of insoluble and infusible The reaction involved which gives rise to the insolubility and infusibility of these substituted polyamides is apparently one involv ing cross-linkage of the linear polyamide chains by means of the reactive N-methylol groups. The susceptibility of the N-alkoxymethyl polyamides as obtained by the above mentioned reaction to undergo this change thus precludes their use in many plastic applications.

It is an object of this invention to provide improved heat stable synthetic linear N-alkoxymethyl polyamides. A further object is to provide a process for the preparation of N-alkoxymethyl polyamides which possess improved melt stability. Other objects will appear hereinafter.

These objects are accomplished by the treatment of the previously described N-alkoxymethyl polyamides containing substantial amounts of N- methylol groups with group consisting of alkali metalearth-hydroxides, carbonates, or sulfites.

8,441,007 4 The removal of the methyiol group whose preamethyl polyamide which contained methylol once has been found to induce heat instability groups was not fusible when heated at 218 C. of the polyamide, can be readily carried out to obfor 10 minutes, the alkali treated N-methyloltain an N-alkoxymethyl polyamide of improved free polyamide fused readily at this temperature.

heat stability suitably by heating an aqueous slurry of the N-alkoxymethyl polyamide from which it is desired to remove N-methylol groups with an aqueous solution or suspension of an alkali metalor alkaline earth-hydroxide, carbonate, or sulfite. After heating such a slurry at a temperature of up to 100 C. with the alkali metalor alkaline earth-compound which may be present in quantities as low as 0.1% or less based on the total weight of the slurry for a period of approximately 5 to minutes or longer, the polyamide is filtered and washed with water. The N-alkoxymethyl polyamide thus formed is found to be free of methylol groups. The methylol-free N-alkoxymethyl polyamide can be readily molded, calendered, or melt spun and in general the polymer thus possesses improved heatand melt-stability.

This invention is further illustrated by the following examples in which the parts are by weight.

Example I A slurry was prepared which consisted of 10 parts of water and 1 part of finely divided granular N-methoxymethyl polyhexamethylene adipamide formed by the action of formaldehyde and methanol on polyhexamethylene adipamide of intrinsic viscosity of about 1.0. To this vigorously stirred mixture there was added 0.01 part of sodium sulfite and the slurry heated at 60 C. (or 30 minutes. The resulting modified polyamide was then filtered, washed thoroughly with water and the polyamide was then dried. This polyamide was found to contain no methylol groups after this treatment although initially it had a methylol content of 1.5%. The methoxyl content which was approximately 7.0% was eslentially unchanged.

The dried methyiol-free polyamide was found to be slightly less soluble in organic solvents and less sensitive to water than the original N-methoxymethyl polyamide. The polymer thus obtained was easier to mold and did not produce an insoluble infusible polymer upon heating at temperatures of about 200 C.

The initial N-methoxymethyl polyhexamethylene adipamide used in the aboveexample was obtained by adding 1 part paraformaldehyde dissolved in 1.3 parts methanol to a solution of 1 part by weight polyhexamethylene adipamide in 3.7 parts formic acid and maintaining the mixture at 60 C. for 30 minutes when 0.3 part additional methanol was added. The reaction product was precipitated by pouring into a mixture of acetone and water.

Example II One part of N-ethoxymethyl polyhexamethylene adipamide obtained with ethyl alcohol in a manner similar to that given above and having an ethoxyl content of 7.4% and a methylol content of 1.7% was added to 100 parts of water solution which contained 10 parts of sodium hydroxide. This mixture was stirred vigorously and heated at approximately 100 C. for 10 minutes. The modified polyamide was then filtered and washed thoroughly with water to remove all traces of alkali. The dried N-ethoxymethyl polyamide wasthen found to contain no methylol groups while the ethoxyl content was slightly increased. Although the original N-e'thoxy- Example III Ten parts of N-ethoxymethyl polyhexamethylene adipamide containing 0.26% methylol groups and 8.0% N-ethoxymethyl groups was heated one hour at 60 C. with a 1% aqueous suspension of calcium sulfite. After washing and drying the polymer, it was then found to contain only 0.03% methylol.

A further modification that has been found useful to improve the melt stability of the alkoxymethyl polyamide through the N methylol removal has been to introduce the reagent effecting the removal of the N-meth'ylol groups to the N-alkoxymethyl polyamide during milling or calendering of the polyamide. Such a treatment has been found to prevent insolubilization and to result in a polymer which does not set up to an infusible material upon repeated milling or calendering. This treatment is conveniently carried out by the addition of the reagent brin ing about the removal of the N-methylol groups either in an aqueous solution or as a solid to the substituted polyamide to which water (in an amount roughly equivalent to the weight of poly amide) has been added. The milling of such a mixture is carried out for approximately onehalf hour at temperatures of from room temperatuieto C. The reagent effecting the removal of the N-methylol groups, e. g., sodium sulfite or sodium hydroxide, is customarily employed in this operation in amounts of about 10- 20% by weight based on the weight of polyamide.

Alkali metal hydroxides, carbonates and sulfites are especially useful in the removal of the N-methylol groups. These include NaOH, NazCOa, NazSOs, KOH, KzCOs, K2303, and the corresponding lithium, rubidium and cesium compounds. The alkaline earth hydroxides, sulfites and carbonates, e. g., Ca(OI-I)2, Ba(OH)2, Sr(OH)z, CaSOs, 39.803 and SICOs may also be used in the removal of the N-methylol groups by the method illustrated in the examples. It is preferred that the solubility of the alkali or alkaline earth compound be greater than 0.004 g. per 100 ml. of water at 20 C. It is also preferredth'at the reagents selected for removing the N-methylol groups have a pH of at least 7.5 as measured in a 1% aqueous solution or suspension. Quaternary ammonium hydroxides, amines, e. g., ethylamine, dimethylamine, etc., basic salts, such as NaaPO4, NazS, NazBio'z, and the alkaline earth compounds which are operable in this reaction usually require a longer reaction time to effect removal of N-methylol groups than for the alkali metal compounds. The amount of these materials can be varied within wide limits. Concentrations of from less than 0.1% to 10% or more of the hydroxides, sulfites, or carbonates in aqueous solution are usually used. These reagents when employed at temperatures of up to 100 C. for several hours have been found not to degrade the polymeric amide. This is to be contrasted to the use of mineral acids in dilute solutions such as phosphoric, sulfuric or hydrochloric to bring about the removal of N-methylol groups from the synthetic linear N-alkoxymethyl polyamide. When these latter reagents are used, the N-alkoxymethyl polyamide undergoes partial hydrolytic degradation and furthermore the N-alkoxymethyl group may be partially or completely removed along with the removal of the N-methylol groups.

With alkali sulfites in aqueous solution, for the removal of methylol the order of is meric material inwater.

Furthermore, the N-meth'ylol groups can be suitably removed from the N-alkoxymethyl polyamicles containing N-methylol groups by adding The N-methylol-free N-alkoxymethyl polythis invention can ticularly as elastic fibers, films, or in molded Particular advantage results in the employment of these polymers in calendering or in bonates, and sulfltes, and continuing other operations such and melt As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it

from methylol groups, said carbonamide being the polymer obtained by converting amide groups of a synthetic linear polycarbonamide, which has an wherein OR is an alkoxy group.

2. A process for removing N-methylol groups from an N-alkoxymethyl poiycarbonamide containing such groups, heating at a temperature of from 60 C. an aqueous slurry of said N-alkoxymethyl N-alkoxymethyl polycarbonamide being the polyro p wherein OR is an alkoxy group.

3. A process for removing N-methylol from an N-aikoxymethyl polycarbonamide con- C. an alcoholic solution of said N-alkoxymethyi of the group consisting of alkali metal hydroxides, carbonates and sulfltes, hydroxides, carthe heatin koxymethyl polycarbcnamide is said compound is an alkali 'metal hydroxide,

substantially free from methylol groups, said N- alkoxymethyl poiycsrbonamide being the polymer obtained by converting amide groups said compound-is an alkali metal carbonate. said compound is an alkali metal sulflte.

u n v V s '1. The process set iorth in claim 1 m which said compound issodium sulnte. in the polymer chain of a synthetic linear poly- THEODORE LE SUEUR CAIRNS.

carbonamide, which has an intrinsic viscosity of 1 at least 0.4 and in which the average number of E E ENCES CITED carbon awms Separating said amide gmups is The following references are of record inthe at least two, to groups of the formula me of this patent:

1 UNITEDSTATES PATENTS u Number Name Date wherein OR is an alkoxy group. 15 2,201,741 Owens et 8'1 M y 21, 1940" 4. The process set forth in claim 1 in which 2.24 .184 Austin et a1. June 3, 1911 2,288,752 Mighton June 18, 1942 A 5. The process set forth in claim 1 in which 6. The process set iorth in claim 1 in which 

